Punch presses having rotatable upper and lower turrets are well known in the machine tool art. The upper turret carries a plurality of punch tools of various sizes and shapes, while the lower turret carries a plurality of dies which correspond to the tools carried by the upper turret. Rotation of the turrets under control of a CNC brings a selected pair of tools and dies into registry under the ram of the punch press. Various machining operations are performed by these tools including punch shaping, shearing, through-hole punching, and nibbling operations, among others.
Turret punch presses are typically employed in high volume, high speed manufacturing systems including those systems used for performing the above-described machining operations and for manufacturing sheet metal components from metal sheet stock. The punches which are carried by the turrets of these machines are generally driven by hydraulically- or mechanically-operated rams. Initially, a sheet workpiece is placed on the work table of the turret press and a selected punch, which is supported and manipulated by the turret, is delivered into a generally vertical position over the workpiece and underlying die. A striker driven by the ram imparts a sharp blow to one end of the punch, causing the shaped opposite end of the punch to rapidly and forcefully impact the sheet material, thereby punching a hole or creating a shaped contour in the sheet material having a desired configuration roughly corresponding to the configuration of the shaped punch end.
Common to the punching process, however, are a number of well known problems associated with the punching process which may compromise the integrity of the resulting punched hole or contour in the sheet material. For example, micro-cracks formed at the periphery of the punched hole may propagate into the surrounding sheet material when subjected to fatigue loading caused by subsequent machining operations or after assembly in a finished product. It is known that these micro-cracks when exposed to fatigue loading may grow and extend even further into the sheet material, further compromising the structural integrity of the component and the assembly in which it is incorporated. One approach to alleviating this punching problem is the use of a router to shape the periphery of the punched hole or contour or to machine away the micro-cracked periphery thereof.
Another well known punching problem is slug blowout caused during punching, resulting in a generally ragged hole having an taper increasing in diameter through the sheet material in the direction of the initial stroke of the punch. The punched hole, which is partially formed by a shear mechanism, cannot be used for close tolerance applications due to non-uniform through-hole variations and center-to-center variations between punched holes. One current solution to this problem is reaming or drilling the punched hole as necessary to remove the ragged inner diameter of the hole, thereby creating a uniform inner diameter through the thickness of the sheet material.
A further problem with current punching processes, and especially for orthogonal shearing processes, is deviation in sheet edge linearity which is a cause of inaccurate bending as a result of imprecise backgaging (i.e., orientation of the sheet on the X-axis). Yet another problem associated with punching is a resulting punched contour having a rough, scallop-like edge which may also detrimentally affect hole dimensioning and center-to-center accuracy relative to adjacent punched contours in the punched component and adjacent components of the resulting assembly.
Various secondary operations are commonly employed to remedy these punching problems including filing or grinding of sheet edges after punching, reaming or drilling of resulting punched holes, or filing or grinding of the punched holes or contours. These secondary machining operations are frequently used in the precision sheet metal and aerospace industries as necessary to accomplish precision punched and oriented through-holes and contours as required. In order to provide these secondary operations, however, the overall machining process must be interrupted, whereby the punched component or sheet must be relocated to a separate secondary machining facility subsequent to the initial punching operation. Continuity of production is disrupted as a result of this physical relocation of the unfinished sheet material workpiece, thereby compromising production efficiency. Furthermore, the workpiece is subjected to cumulative gauging and machining errors resulting from a necessary repositioning of the workpiece in a second clamping and positioning device. Finally, redundant and costly secondary machining equipment is required to perform those machining operations previously described, according to the prior art.